As power electronic devices utilize more power and consequently dissipate more heat, efficient cooling of such power electronic devices is important. One way of providing an efficient cooling system for such power electronic devices, for example, semi-conductor switching elements, is providing a two-phase cooling circuit. Such a cooling circuit brings a liquid into thermal contact with a device emitting heat. The liquid is heated by dissipated heat of the device and reaches a boiling temperature. As the temperature of the liquid itself will not rise above the boiling temperature, the temperature of the liquid and therefore the temperature of the electronic device is kept at a temperature of the boiling point of the liquid as a maximum.
The liquid is stored in a reservoir inside an evaporator. The evaporator is in thermal contact with the heat emitting device. The vapor of the liquid is converged through a conduit to a condenser. Within the condenser the vapor is changed into liquid by rejecting heat. For example, in the condenser the heat is rejected to a coolant fluid, such as air at ambient temperature. The vapor thus returns to its liquid phase. The condenser and the evaporator are connected via a second line in order to feed back the condensed vapor as liquid again to the liquid reservoir of the evaporator.
Such a cooling device is disclosed in U.S. Pat. No. 5,195,577. A problem of such a cooling circuit can be that the evaporator at the same time provides the function of a liquid reservoir. Thus, the cross-section of such an evaporator can be relatively large. Consequently, the efficiency of the evaporator can be low because the introduced heat leads to boiling of the liquid which is provided in a large volume of the evaporator. This so-called “pool boiling” has poor heat transfer performance, is bulky, requires a large fluid inventory, and can be difficult to make leak proof at high pressure.
To improve the heat transfer performance of an evaporator it has been considered to use the so-called convection-boiling. In order to achieve the convection-boiling effect the cross section of the evaporator is reduced. Due to the reduction of the cross section of the evaporator a mixture of a gas phase and the liquid phase at the exit of the evaporator flows to the condenser. By introducing the vapor mixture to the condenser with the vapor containing liquid droplets, the performance of the condenser can be decreased. So the positive effect of reduction of the cross section area of the evaporator can be reduced to a large extent by the poor heat transfer performance of the condenser.
Thus, there is a need for an evaporator for a cooling circuit with an improved heat transfer without affecting the performance of a condenser of the cooling circuit. It is desirable that there is sufficient liquid phase cooling fluid available in the evaporation channels so that the evaporator does not run dry during operation.